Thomas W. Donnelly, 1966. "GEOLOGY JOHN, OF ST. THOMAS AND ST. JOHN, U. S. VIRGIN ISLANDS", Caribbean Geological Investigations, H. H. Hess, Carl O. Bowin, Thomas W. Donnelly, John T. Whetten, E. R. Oxburgh
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The oldest rock unit of the northern Virgin Islands is the ?Lower Cretaceous Water Island Formation (10,000–15,000 feet plus), which consists of keratophyre flows and tuffs, spilite flows, and radiolarian tuffs. Terrigenous sediments are absent, and the entire volcanic sequence appears to have been extruded on the ocean floor.
Overlying the Water Island Formation with a mild angular unconformity is the Virgin Island Group, consisting of the Louisenhoj Formation, Outer Brass Limestone, Tutu Formation, and Hans Lollik Formation, all of possible Albian age. The Louisenhoj Formation (8000–13,000 feet maximum) consists of augite andesite tuff and volcanic breccia with intercalated conglomerate (Cabes Point Conglomerate lithofacies) containing pebbles and cobbles of Water Island Formation lithologies. The eruptive center during Louisenhoj time was probably located close to Pillsbury Sound, between St. Thomas and St. John. In western St. John and eastern St. Thomas this formation consists of coarse cone debris derived largely from a postulated sub-aerial cone. In central and western St. Thomas pyroclastic rocks, frequently showing slumping, predominate.
The Outer Brass Limestone (200–600 feet) consists of thin-bedded, carbonaceous, radiolarian, tuffaceous limestone with intercalated tuff beds.
The Tutu Formation (4000–6000 feet exposed) is a relatively coarse volcanic wacke which consists almost entirely of unweathered Louisenhoj debris with a small quantity of limestone fragments. The Coki Point Megabreccia lithofacies is an intercalated slumped mass of andesite and limestone blocks, the latter containing a neritic fauna of corals, gastropods, pelecypods (including the rudist Caprinuloidea), and echinoids, and is of apparent Albian age. The Congo Cay Limestone, near the top of the Tutu Formation, crops out on only one of the American Islands.
The poorly exposed Hans Lollik Formation (6000–10,000 feet exposed) is the youngest unit present and lithologically resembles the Louisenhoj Formation.
Late Cretaceous or early Tertiary deformation caused mild folding, with dips averaging 40° north. There are two prominent sets of strike-slip faults: a dextral one trending northwest, and a sinistral one trending northeast. There is also a north-south set of normal faults. The sinistral strike-slip faults in many cases have a considerable normal component, and appear to parallel the principal fault of the Anegada Trough.
Late intrusive rocks include dioritic plutons (with minor acid differentiates), quartz-andesine porphyry, andesine-hornblende porphyry, lamprophyre, breccia dikes, and rare pegmatites. The emplacement of the plutons preceded much of the major faulting, but the late dikes followed this faulting and were commonly emplaced along fractures parallel to the fault planes.
Contact-metamorphic aureoles of hornblende hornfels and probably pyroxene hornfels facies surround the plutons.
Keratophyre magma is believed to have been a low-melting fraction of hydrated mantle material which was depressed during compressive deformation. Spilite, augite andesite, and diorite magmas were very similar and represent more completely fused mantle. The contrasting mineralogy of the spilite and augite andesite is thought to have originated from contrasting environments of extrusion. Whereas the spilite was extruded abyssally and the volatiles were largely retained until most of the heat loss had taken place, the augite andesite was erupted subaerially and the volatiles had separated from the magma at a high temperature well within the orifice of extrusion.